1. Field of the Invention
The present invention generally relates to a focus detecting optical system for use in a camera, and more particularly, to a focus detecting optical system employed as a focus detecting device in a single lens reflex camera or a video camera provided with automatic focus adjustment.
2. Description of the Prior Art
FIG. 1 depicts a focus detecting optical system employing therein the conventional phase difference detecting method. The focus detecting optical system of FIG. 1 is comprised of a condenser lens Lo and a pair of image re-forming lenses L1 and L2 positioned substantially symmetrically with respect to an optical axis lo (referred to as the main optical axis hereinafter) of an objective lens (not shown). An image formed by the objective lens is formed again as first and second images by the condenser lens Lo and the paired image re-forming lenses L1 and L2. The distance between the first and second images re-formed by the image re-forming lenses L1 and L2 varies in accordance with a state of focus adjustment of the objective lens. Accordingly, if first and second rows I and II of a plurality of light receiving elements of a line sensor Po are arrayed in a single line at a location, or in the vicinity thereof, conjugate to a predetermined image plane FP with respect to the condenser lens Lo and the paired image re-forming lenses L1 and L2, the first and second rows I and II of the light receiving elements can detect a change of the position of the first and second images, thus enabling the detection of the state of focus adjustment of the objective lens.
In FIG. 1, the length S of a focus detection area on the predetermined image plane FP of the objective lens is determined on the basis of the lengths S.sub.I and S.sub.II of the first and second element rows I and II of the line sensor Po, when an image magnification of the optical system is constant. Accordingly, for the purpose of lengthening the length S of the focus detection area, the lengths S.sub.I and S.sub.II are required to be lengthened.
FIG. 3 depicts an example of a focus detecting optical system in which the length S of the focus detection area is lengthened, as compared with the system of FIG. 1. The lengthened S.sub.I and S.sub.II of the element rows I and II involve the necessity of extending the distance between the first and second images to be re-formed. In the optical system of FIG. 3, the distance l between the centers O and O' of the paired image re-forming lenses L1 and L2 is extended. Thus, optical paths for re-forming the first and second images are deflected from those shown by dotted lines to those shown by double dotted chain lines so that the distance between the first and second images may be extended. The distance between a pair of apertures A1 and A2 formed in an aperture mask AM may conceivably be extended as well. In this case, however, since light fluxes for detecting the focus are liable to be vignetted, F.No. of interchangeable lenses enabling the focus detection is restricted. Accordingly, in this embodiment, only the distance between the image re-forming lenses L1 and L2 is extended from that shown by a dotted line to that shown by a solid line, thus extending the distance between the first and second images.
However, if the focus detection area is enlarged only by the off-centered image re-forming lenses L1 and L2, image planes of the first and second images to be re-formed are greatly curved. This fact disadvantageously involves a focus detection error, since point images formed on the first and second element rows I and II can not become symmetric in size with respect to the center of a light receiving portion.
FIGS. 2 and 4 depict the size of the point images on the element rows I and II of FIGS. 1 and 3, respectively. In FIG. 1, the distance l between the image re-forming lenses L1 and L2 is determined so that the light fluxes from the intersection C between the predetermined image plane FP and the main optical axis lo may travel substantially straightforwardly to form respective images on the central points C.sub.I and C.sub.II of the first and second element rows I and II. In this case, there is produced little difference in size between the point images, of end points A and B in the focus detection area, re-formed on end points A.sub.I, A.sub.II and B.sub.I, B.sub.II of respective element rows I and II, as shown in FIG. 2. In contrast, FIG. 3 shows the case in which the light fluxes from the intersection C are deflected in the vicinity of the image re-forming lenses L1 and L2 and the distance l therebetween is extended so that the light fluxes may be re-formed on the central points C.sub.I and C.sub.II of the element rows I and II. In this case, the light flux entering the image re-forming lens L1 from the point A can enter the first element row I without being subjected to any large deflection when passing through the image re-forming lens L1. To the contrary, the light flux entering the image re-forming lens L2 from the point A enters the second element row II while being subjected to relatively large deflection when passing through the image re-forming lens L2. Accordingly, the light flux having passed through the image re-forming lens L2 forms an image a little ahead of another image formed by the light flux having passed through the image re-forming lens L1. In other words, the point image re-formed on the end point A.sub.II of the second element row II becomes smaller than that reformed on the end point A.sub.I of the first element row I. Accordingly, as shown in FIG. 4, there is produced considerable difference in size between the point images, of the end points A and B of the focus detection area, reformed on the end points A.sub.I, A.sub.II and B.sub.I, B.sub.II of the first and second element rows I and II. This causes the focus detection error.
FIG. 5 depicts an object having three white stripes X, Y and Z on the black background. In a focus detection area F.sub.A, the white stripes X and Z are placed close to the end points A and B, respectively, whereas the white stripe Y is placed in the vicinity of the central point C. FIG. 6 depicts outputs of the first and second element rows I and II in this case. As clearly shown in FIG. 6, the first and second element rows I and II view the white stripe Y in substantially the same width in the vicinity of the central point C and the white stripes X and Z in different widths in the vicinity of the end points A and B as if the two element rows I and II view different white stripes having different widths. Thus, when the size of the same point image is viewed remarkably differently by the two element rows I and II, the degree of coincidence between the first and second images is lowered, rendering the focus detection accuracy to be lowered. This phenomenon becomes more conspicuous when an image point locates farther away from the main optical axis lo. Accordingly, sufficient effect can not be attained by the enlarged focus detection area.